The most prevalent form of a mobile wireless communication system is a cellular network. In such a network, a territory serviced by it is divided into a plurality of geographically substantially distinct, but normally overlapping cells. Within each cell is a base station transceiver subsystem (BTS) at which there is an antenna or antenna array connected to a bank of radio transmitters and receivers (hereinafter "radios") for communicating with mobile radios (phones) within the territory. These radios are controlled by a base station transceiver controller or more popularly shortened to base station controller (BSC). A plurality of these BTSs are connected through the BSC by data and voice links to a mobile telecommunications switching office (MTSO) or also known as a mobile telephone exchange (MTX). This link is often a telephone line having the capacity of a T1 connection for each BTS or a microwave radio link of a different frequency than used between the BTS and the mobile radios. Within the MTX and the BSC there are a plurality of subsystems for routing calls to appropriate BTSs and for issuing instructions to the connected BTSs.
The MTX connects calls between two mobile radios within the network, between the mobile radios and the public switching telephone network ("PSTN") and occasionally between a mobile radio of that system and a radio of a foreign mobile system. As is known by those skilled in the art, there are many different formats used for communicating to the mobile radios such as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) etc. However each MAX and its associated BSCs has, regardless of the communication format, similar internal components in that each needs enough interface ports to communicate with the BTSs, a vocoder capacity capable of providing a given grade of service during periods of maximum traffic, and a capacity sufficient to transmit/receive and to control enough different communication channels at each BTS such that the blocking level and accordingly the given grade of service is maintained.
As is known to those skilled in the art, the signals passing through the switch, on their way to and from the mobile phone are switched in time or space or both by elements of the MTX. The incoming signals, after passing through the switch are passed through vocoders in a base station controller (BSC) on the way to one of the channel elements in a base station. A single base station may be connected to more than one interface port.
Typically the design of at least the vocoder portion of a BSC is modular so that when system signal handling capacity is increased, the capacity is increased in incremental blocks of vocoders. In other words the number of vocoders in a selector bank (part of a traffic switch in a BSC) module might be 12. If 13 vocoders were required, then two modules would be required, providing 11 unnecessary selector elements. The design of the base stations connected to the system is also modular at least in the portion which may contribute to signal blocking. Further, a channel element module might contain 8 channel elements as opposed to the 12 vocoders in the BSC. This modular situation acts to cause system costs to change in a non-linear manner with changes in blocking probability. It should also be realized that, if the system is "over-designed" to insure that the customer designated grade of service is obtained, it will also be "over costly".
Call blocking in the wired portion of a cellular network primarily occurs due to the lack of channel elements (typically, lack of channel cards) at the BTS and/or vocoders (comprising a part of a SBS (Selector Bank Subsystem) block in a given specific system) portion of a BSC. Given a total blocking probability BP for the wired network portion of the system by the customer or someone responsible for total system design, a system designer must distribute the blocking between the SBSs and the BTSs. As previously mentioned, deployment of these resources is typically imprecise, due to the modular deployment of the channel and selector elements. Thus an attempt at optimum design or allocation of assets to obtain a minimum cost while staying within required design blocking parameters has typically entailed luck, experimentation and guesswork.
The average busy hour cell traffic is typically specified to the system designer or it can be measured when the design is for the purpose of upgrading an existing system. Most system designs use the same average busy hour traffic for all cells. As is known to those skilled in the art, cell traffic depends upon many variables including the network topology, the number of users in the cell, as well as in the neighboring cells, and the frequency of handoff occurrences. A handoff occurs when the mobile radio is moving from one cell to another with respect to geographical location as well as with respect to the radio environment or RF environment. Movement for a handoff with respect to the "radio environment" or "RF environment" involves the mobile radio going into a fading condition. Therefore, the words "moving" and "distance" in this discussion refer to changing not only the geographical position but also the RF environment. The term "handoff" refers to the transferring of communications from the mobile customer by the MTX or the BSC from one cell BTS to another cell BTS. In many older systems, the switching operation involves the mobile phone receiving information as to the frequency to be used in connecting to the new BTS, changing to the new frequency (thus causing an immediate disconnection from the old BTS), and establishing a connection with the new BTS before the call can continue. CDMA systems can actually allow the mobile phone to be connected to the new BTS before being disconnected from the old BTS and thus a term was coined of "soft handoff" or SHO for this type of handoff. When it is obvious to which adjacent cell a mobile radio is being transferred, it is referred to as a "two way handoff." However, at times, a mobile radio approaches the influence of two adjacent cells simultaneously, and, thus, both of the affected BTSs must be prepared to receive the incoming mobile radio depending upon the direction that the mobile radio takes upon leaving a given cell. This handoff is designated in the prior art as a "three way handoff." Once the mobile customer is communicating with another cell, the communication channels reserved in the other two cells are released for reuse by other customers.
Call blocking in any telephone system is primarily due to failure of a communication link or unavailability of dedicated services to support the call. The blocking experienced in any mobile telephone system is from two different media. One is the area interface, or RF blocking, and the other is the wired cellular network or network blocking. Usually, the purchaser of a system intends to provide a given total end-to-end blocking grade of service. The designer of the system being delivered to the purchaser distributes this end-to-end blocking probability between the area interface and the network itself.
RF blocking probability is defined as the probability that a call (originating from or terminating at a mobile radio in a cell) gets blocked at the area interface between the mobile radio and the BTS or base station. The blocking probability at the area interface depends heavily on the changing RF propagation environment in the coverage area of each sector or cell. It also depends upon the location of the mobile radio with respect to that of the individual sector or cell antennas. It is possible that a mobile radio may be well within the coverage area of a cell but may have a best RF path to an entirely different cell in the network due to holes in the coverage of the first mentioned cell. RF blocking probability is typically estimated during the RF planning process and is updated with results obtained from field tests. The field tests will often define the holes in the coverage.
A call is typically blocked in the wired network portion of a mobile telephone system by the main switch within the system or an associated base station. There are three primary reasons for this to happen. A first reason is loss of voice or control packets in the packet switch portion of the network. It will be noted, however, that loss of voice packets only affects call quality in most situations. A second reason is the lack of channel elements in a base station, and the third reason is the lack of vocoders in a switch. Some networks use packet switching for both control and voice in the BSC and the BTS; however, the main switch used in conjunction with the present invention preferably uses packet switching for control and circuit switching for voice.
Loss of control packets may occur due to a finite buffer size at the BTS nodes. Buffers and links are typically engineered such that the packet loss probability is much lower than typical blocking probabilities assumed for inadequate numbers of selector elements or channel elements. Thus, in most engineering design considerations, the blocking probability due to the buffer size limitations are typically assumed to be negligible. Cost decisions, therefore, primarily revolve around how to distribute the network blocking probability due to lack of selector and channel elements by ascertaining how to distribute the blocking probability between the BTSs and the SBSs (vocoders) within the BSC.
In the prior art, the design process has been to assume that the total blocking probability BP, for the wired network portion of the system (the MTX, the BSCs and the associated BTSs), should be divided equally between the vocoders and channel elements. Thus the system has been designed to provide enough vocoder capacity so that the vocoder blocking probability does not exceed that BP/2 value and then the number of channel elements in each BTS is arrived at the same way. In other words, if the total wired network blocking were 2.2%, the blocking of the vocoder portion would be such as to not exceed 1.1% during average busy hour traffic and the same would be true of the blocking in the channel elements. While a review of the prior art designs will show that this prior art design assumption has, in many cases, resulted in a lowest cost system design, there are also many instances where a different division of blocking probabilities between the vocoders and channel elements would have resulted in a lower cost system design than was actually obtained.